The present invention relates generally to gas storage and dispensing systems, and particularly to systems of such type utilizing a monolithic carbon sorbent as a gas storage medium.
The physical adsorbent-based gas storage and dispensing system disclosed in Tom et al. U.S. Pat. No. 5,518,528 has revolutionized the transportation, supply and use of hazardous gases in the semiconductor industry. The system includes a vessel holding a physical adsorbent medium such as molecular sieve or activated carbon, having sorptive affinity for the gas that is to be stored in and selectively dispensed from the vessel. The gas is held in the vessel in an adsorbed state on the sorbent medium at reduced pressure relative to a corresponding empty (of sorbent) vessel holding an equivalent amount of gas in the xe2x80x9cfreexe2x80x9d (unadsorbed) state.
By such reduced pressure storage, the safety of the gas storage and dispensing operation is substantially improved, since any leakage will result in a very low rate of egress of gas into the ambient environment, relative to a conventional high pressure gas storage cylinder. Further, the low pressure operation of the adsorbent-based system is associated with a lower likelihood of such gas leakage events, since the reduced pressure reduces the stress and wear on system components such as valves, flow controllers, couplings,joints, etc.
In such adsorbent-based gas storage and dispensing systems, the working capacity of the physical adsorbent medium is an operating constraint. The working capacity is the amount of gas that can be stored (xe2x80x9cloadedxe2x80x9d) on the sorbent medium and desorptively removed from such sorbent medium for use. The working capacity is a function of the storage pressure of the gas in the sorbent medium-containing gas storage vessel, and the dispensing condition of the desorbed gas (e.g., dispensing pressure of the desorbed gas, when pressure differential is used to effect desorption, and temperature levels of respective storage and dispensing conditions, when thermal desorption of gas is used as the dispensing modality), and the type and character of the sorbent medium itself (e.g., involving such parameters as sorbent media size, shape, porosity, pore size distribution, and tortuosity of interior pore passages).
The art is continually seeking improvement in working capacity of the physical adsorbent-based gas storage and dispensing system.
The present invention relates to physical adsorbent-based gas storage and dispensing systems, and to an improved working capacity system of such type.
In one aspect, the present invention relates to a fluid storage and dispensing apparatus, comprising a fluid storage and dispensing vessel having an interior volume, wherein the interior volume contains a physical adsorbent sorptively retaining a fluid thereon and from which the fluid is desorbable for dispensing from the vessel, and a dispensing assembly coupled to the vessel for dispensing desorbed fluid from the vessel, wherein the physical adsorbent comprises a monolithic carbon physical adsorbent that is characterized by at least one of the following characteristics:
(a) a fill density measured for arsine gas at 25xc2x0 C. and pressure of 650 torr that is greater than 400 grams arsine per liter of adsorbent;
(b) at least 30% of overall porosity of said adsorbent comprising slit-shaped pores having a size in a range of from about 0.3 to about 0.72 nanometer, and at least 20% of the overall porosity comprising micropores of diameter  less than 2 nanometers; and
(c) having been formed by pyrolysis and optional activation, at temperature(s) below 1000xc2x0 C., and having a bulk density of from about 0.80 to about 2.0 grams per cubic centimeter.
Another aspect of the invention relates to a method of forming a monolithic adsorbent for use in a gas storage and dispensing system, said method comprising: molding a pyrolyzable material into a monolithic shape; and pyrolyzing the pyrolyzable material under pyrolysis conditions producing a monolithic adsorbent that is characterized by at least one of the following characteristics:
(a) a fill density measured for arsine gas at 25xc2x0 C. and pressure of 650 torr that is greater than 400 grams arsine per liter of adsorbent;
(b) at least 30% of overall porosity of said adsorbent comprising slit-shaped pores having a size in a range of from about 0.3 to about 0.72 nanometer, and at least 20% of the overall porosity comprising micropores of diameter  less than 2 nanometers; and
(c) a bulk density of from about 0.80 to about 2.0 grams per cubic centimeter, wherein the aforementioned pyrolysis conditions comprise temperature below 1000xc2x0 C.
A further aspect of the invention relates to a method of storing and dispensing a gas, comprising: fabricating a gas storage and dispensing vessel; disposing a physical adsorbent in the vessel having sorptive affinity for said gas; charging said gas to said vessel for adsorption on the physical adsorbent; sealing the vessel with a valve head containing an actuatable valve, to enclose the physical adsorbent and adsorbed gas, and isolate same from an exterior environment of the vessel; desorbing the adsorbed gas from the physical adsorbent, and actuating the actuatable valve in the valve head, to flow gas from the vessel and through the actuatable valve, for gas dispensing; wherein the physical adsorbent is characterized by at least one of the following characteristics:
(a) a fill density measured for arsine gas at 25xc2x0 C. and pressure of 650 torr that is greater than 400 grams arsine per liter of adsorbent;
(b) at least 30% of overall porosity of said adsorbent comprising slit-shaped pores having a size in a range of from about 0.3 to about 0.72 nanometer, and at least 20% of the overall porosity comprising micropores of diameter  less than 2 nanometers; and
(c) a bulk density of from about 0.80 to about 2.0 grams per cubic centimeter, wherein said pyrolysis conditions comprise temperature below 1000xc2x0 C.
Other aspects, features and embodiments of the present invention will be more fully apparent from the ensuing disclosure and appended claims.